In designing x-ray systems for producing images of human subjects, one of the primary considerations is reducing the exposure to radiation which is experienced by the patient. In computerized tomography x-ray scanning systems, there is generally an x-ray source and an array of radiation detectors which are both movable relative to the body being imaged. As the x-ray source and detector move relative to the patient, a large number of x-ray images are taken, and the data from these images is processed to provide a representation of the cross-sectional density of the subject.
In known tomography scanners, the radiation level provided by the x-ray source for each exposure is selected prior to the beginning of the x-ray scan. The particular exposure value chosen is dependent on the patient's size and on the section of the body being examined, and the exposure is not changed as the scanner rotates about the patient. Since attenuation through the body is significantly different for views taken from different angles, the use of a single preselected exposure value results in the patient being exposed to more radiation than is needed for exposures at most angles. Additionally, in conventional systems, the exposure settings are determined using tables based on typical size and weight characteristics of patients, and these tables do not always produce an optimum exposure selection for actual patients.